The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years. In these applications, the coating material is discharged in atomized form, and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged atomized particles. In the past, coating materials of the solvent-based variety, such as varnishes, lacquers, enamels, and the like, were the primary materials employed in electrostatic coating applications. The problem with such coating materials is that they create an atmosphere which is both explosive and toxic. The explosive nature of the environment presents a safety hazard should a spark inadvertently be generated, such as by accidentally grounding the nozzle of the spray gun, which can ignite the solvent in the atmosphere causing an explosion. The toxic nature of the workplace atmosphere created by solvent coating materials can be a health hazard should an employee inhale solvent vapors.
As a result of the problems with solvent-based coatings, the recent trend has been to switch to water-based coatings which reduce the problems of explosiveness and toxicity. Unfortunately, the switch from electrostatically spraying solvent-based coatings to those of the water-based type has sharply increased the risk of electrical shock, which risk was relatively minor with solvent-based coatings. The risk of electrical shock is occasioned in the use of water-based coatings due to their extreme electrical conductivity, with resistivities of such water-based coatings often falling within the range of 100 to 10,000 ohm centimeters. This is in contrast to resistivities of 200,000 to 100,000,000 ohm centimeters for moderately electrically conductive coatings such as metallic paint, and resistivities exceeding 100,000,000 ohm centimeters for solvent-based lacquers, varnishes, enamels and the like.
The relative resistivity of the coating material is critical to the potential electrical shock which may arise during an electrostatic coating operation. With coating materials which are either not electrically conductive or only moderately electrically conductive, the column of coating material which extends from the charging electrode at the tip of the coating dispenser through the hose leading back to the supply tank has sufficient electrical resistance to prevent any significant electrostatic charging of the material in the supply tank or the tank itself. However, when coating material is highly electrically conductive, as are water-based coatings, the resistance of the coating column in the supply hose is very low. As a result, a high voltage charging electrode located in the vicinity of the nozzle of the coating dispenser electrostatically charges not only the coating particles, but the coating material in the hose, the coating material in the supply tank and the supply tank itself. Under these circumstances, operating personnel inadvertently coming into contact with an exposed supply tank or a charged hose or any other charged part of the system risk serious electrical shock unless such equipment is grounded to draw off the electricity. If the equipment is indeed grounded at any point, however, the electrostatics will not function because the high voltage charge would be conducted away from the coating dispenser electrode as well.
One of the methods for reducing the electrical shock problem is disclosed, for example, in U.S. Pat. No. 3,971,337 to Hastings which is owned by the same assignee as this invention. The Hastings patent discloses an apparatus for electrostatically isolating the supply tank which is connected to the coating dispenser. While this device is satisfactory for batch operations, it does not readily lend itself to continuous painting lines, i.e., applications wherein an essentially continuous supply of coating material must be provided over a period of time.
This problem has been addressed in apparatus of the type disclosed, for example, in U.S. Pat. No. 4,313,475 to Wiggins. In apparatus of this type, a "voltage block" system is employed wherein electrically conductive coating material is first transmitted from a primary coating supply into a transfer vessel which is electrically isolated from the spray gun. When filled with coating material, the transfer vessel is first disconnected from the primary coating supply and then connected to an inventory tank, which, in turn, is connected to one or more coating dispensers. The coating material is transmitted from the transfer vessel into the inventory tank to fill the inventory tank with a supply of coating material for subsequent transfer to the coating dispensers. While the inventory tank supplies the coating dispensers with coating material, the transfer vessel is disconnected from the inventory tank and connected back to the primary coating supply to receive another quantity of coating material so that the coating operation can proceed essentially continuously.
An important feature of apparatus of the type disclosed in the Wiggins U.S. Pat. No. 4,313,475 is that a voltage block or air gap is provided at all times between the primary source of coating material and the electrically charged coating dispensers. One potential operational problem with the Wiggins design is that separately actuated transfer devices, e.g., pneumatic cylinders or the like, are employed to interconnect the transfer vessel with the primary coating supply, and then to connect the transfer vessel with the inventory tank. Because the two pneumatic cylinders or other transfer devices are actuated independently of one another, it is possible that a malfunction of the controller for such cylinders could result in the connection of the transfer vessel to the primary coating supply at the same time the inventory tank is connected to the transfer vessel. As discussed above, the low resistivity of water-based coating materials can result in the transfer of a high voltage electrostatic charge from the coating guns, through a column of coating material to the primary coating supply, thus creating a hazard of electrical shock.
Another problem with apparatus such as disclosed in Wiggins U.S. Pat. No. 4,313,475 involves the leakage and/or drippage of coating material during the transfer process. As described above, the transfer vessel receives a supply of coating material from the primary coating supply, disengages the coating supply and then engages the inventory tank to transfer the coating material therein for supply to the coating dispensers. In the course of this transfer operation, the transfer vessel must make and break connections at both the primary coating supply and the inventory tank in order to effect the transfer of the coating material. It has been found that the connections and/or valving arrangements employed in such apparatus are susceptible to leakage and/or drippage, and thus present clean-up problems. In addition, leakage of such connections can result in grounding and thus loss of voltage in the electrostatic coating dispensers, and also could create an electrical shock hazard should a stream of dripping coating material contact an ungrounded object which can be touched by the operator.
Other potential operational problems with apparatus of the type disclosed in the Wiggins U.S. Pat. No. 4,313,475 involve handling of the coating material within the system. In such apparatus, the coating material is allowed to pool or come to rest within the transfer vessel and/or inventory tank. The pigments within coating material such as paints tend to settle if allowed to come to rest within a vessel or tank, and apparatus of the type disclosed in the Wiggins patent provide no means of circulating or moving the coating material within either the transfer vessel or inventory tank to maintain the pigments and other solids in suspension.
Another problem with systems of the type disclosed in the Wiggins U.S. Pat. No. 4,313,475 is that when the coating material such as paint is transferred between the vessels and tanks of the Wiggins apparatus, and to the coating dispensers, such movement is obtained by the application of pressurized air within the vessel or tank directly into contact with the coating material to force it from the vessel. An air interface can degrade many types of paints, and it is desirable to avoid contact with air until the coating material is applied to a particular substrate.
One way of avoiding direct air contact with the paint is to employ a piston pump having a cylindrical wall defining a reservoir with a piston movable therein. Air or other operating fluid is applied to one side of the piston which forces paint located on the other side of the piston out of the reservoir. In these types of piston pumps, the piston head is formed with one or more circumferential grooves, each of which carry a seal in a position to slidably engage the walls of the cylinder. While piston pumps of this type avoid the problem of direct contact of air and paint, other limitations have been observed in their operation.
One problem with piston pumps of the type described above is that the seals on the piston head are not effective to completely wipe the cylinder wall clean of paint as the piston reciprocates within the reservoir. As a result, a thin film of paint can form along the cylinder wall which is dried by contact with the operating air introduced into the reservoir as the piston is reciprocated therein. This dried paint leaves an abrasive, high friction residue on the cylinder wall which can create erratic piston motion and lead to premature failure of the seals. Additionally, such paint deposits can get sufficiently tacky or sticky to substantially restrict the motion of the piston, particularly if the system operation is interrupted for a period of time for any reason.
Another problem with piston pumps of the type described above is a phenomenon known as "pressure trap". This condition is caused by a differential rate of wiping of the coating material from the walls of the cylinder where the piston head is provided with two or more circumferentially extending seals which are axially spaced from one another. A reservoir of coating material can build up in the axial space(s) between the seals which forces the seal opposite the pressurized side of the piston against its groove in the piston head. For example, when pressurized air is introduced into the reservoir of the pump on one side of the piston head, the coating material caught within the axial space between the seals is forced in a direction toward the coating material side of the piston, which, in turn, forces the seal closest to the coating material against the lip of the groove in the piston head. When the opposite side of the piston head is pressurized, e.g., upon the receipt of coating material, the coating material captured between the seals is forced in the opposite direction, toward the air side of the piston head, thus causing the seal closest to the air side to be forced against its groove in the piston head. This problem of pressure trap causes additional drag on the system and accelerated seal wear.